A traditional pick-up truck has a cargo box used to carry various cargos. Generally, the cargo box is covered by a tonneau cover which is releasably clamped to, and extended above, the cargo box, so as to protect the carried cargo from adverse weather and improve the aesthetic appearance of the pick-up truck. Tonneau covers can be classified into hard-top type and soft-top type, wherein the hard-top type tonneau cover has a hard top made of rigid material, such as metal, fiberglass or the like, while the soft-top type tonneau cover has a soft top made of flexible material, such as canvas, vinyl plastic or a weather-resistant fabric. Moreover, the tonneau cover can be designed to provide a foldable function to help a user to conveniently extend or store the tonneau cover.
Referring now to FIGS. 1, 2, and 3, schematic views of a traditional hard-top type foldable tonneau cover system for a pick-up truck are illustrated, wherein a tonneau cover system 10 is used to cover a cargo box 21 of a pick-up truck 20, and the tonneau cover system 10 comprises a plurality of frame sections 11, 12, and 13, a plurality of hinges 14, a plurality of rigid cover portions 15 and a plurality of clamp assemblies 16. The number of the rigid cover portions 15 are corresponding to that of the frame sections 11-13 (such as three). The rigid cover portions 15 is made of rigid material, and can be suitably fixed on the corresponding frame sections 11-13. Each of the frame sections 11-13 is constructed by a plurality of frame members. One of the frame sections 11 or 13 can be pivotally connected to another adjacent frame section 12 through two or more of the hinges 14, so that all of the frame sections 11-13 can be adjusted to an extended position or a folded position. The clamp assemblies 16 are generally installed on the lower surfaces of the frontmost and rearmost frame sections 11, 13.
When the frame sections 11 are in the extended position as shown in FIG. 2, the frame sections 11-13 are horizontally extended on the two side walls 22 and a tailgate 23 of the cargo box 21, and fixed thereon by the clamp assemblies 16, so as to cover an inner space of the cargo box 21. When the frame sections 11-13 are converted into the folded position as shown in FIG. 3, the frame sections 11-13 are folded and stacked on a front end of the two side walls 22, so as to expose the inner space of the cargo box 21 for placing or taking cargo.
In the tonneau cover system 10, each of the clamp assemblies 16 mainly comprises a clamp track 161, a block member 162, a handle 163, a rod member 164 and a latch member 165, wherein a proximate end of the clamp track 161 is coupled to a side edge of the frame section 13, and a distal end of the clamp track 161 is supported by a support rail 166 which has two opposite ends coupled to other side edges of the frame section 13; the clamp track 161 has a channel 161a and a pocket 161b located at a distal end of the channel 161a; the pocket 161b having an opening 161c which is closed by a cover plate 161d; the block member 162 is slidably disposed in the channel 161a; the handle 163 and the rod member is pivotally connected to the block member 162, respectively; and the latch member 165 is movably coupled to the rod member 164 and moveable between a clamped position for operatively engaging the side wall 22 of the cargo box 21 and a released position for spacing the latch member 165 from the side wall 22. When the clamp assembly 16 is in a stored position, the handle 163 and the rod member 164 are positioned in the channel 161a, and the distal end of the handle 163 is further extended and retained in the pocket 161b. 
However, there are still some problems existing in the actual use of the structure of the traditional clamp assembly 16, as follows: (1) because the channel 161a only has a limited width, it is difficult and inconvenient for a user to manually operate and pull up the handle 163 and the rod member 164 positioned in the channel 161a during changing the stored position back to the released position and the clamped position; (2) because the channel 161a and the pocket 161b lack tightly positioning structure to ensure the handle 163 and the rod member 164 be tightly positioned therein, there is a risk that the handle 163 and the rod member 164 might finally move out of the channel 161a and the pocket 161b of the rearmost frame section 11 due to vibrations of the moving truck when the frame sections 11-13 are in the folded position and stacked on the front end of the cargo box 21; (3) the clamp track 161 must have a sufficient length to form a channel 161a and a pocket 161b for receiving the handle 163 and the rod member 164, but the longer clamp track 161 might negatively affect the entire strength of the rigid cover portions 15 on the lower surface of the frame sections 11,13.
As a result, it is necessary to provide a clamp storage mechanism to solve the problems existing in the conventional foldable tonneau cover, as described above.